Pulse-amplitude modulation (PAM) is a form of signal modulation and is widely used for transmission of digital data. For example, transmission of digital data over fiber optics often uses PAM. In older 10 Gbit/s (gigabits per second) fiber optic transmission systems, binary signal amplitudes of “0” (light off) or “1” (light on) were used to convey information. Currently, fiber optic transmission systems are transitioning to 400 G to accommodate bandwidth needs. At these transmission rates, PAM-4 is a modulation technique that is commonly used. PAM-4 is a modulation technique whereby 4 distinct pulse amplitudes are used to convey digital information. Amplitude levels 1, 2, 3, and 4 are represented by two bits 00, 01, 11, and 10, respectively. Each pair of bits is called a “symbol”. When one of the four amplitudes is transmitted in a symbol period, there are two bits transmitted in parallel, therefore the data rate is doubled. In other words, PAM-4 modulation is twice as bandwidth-efficient as conventional binary modulation.
In optical fiber communications, a transponder is the element that sends and receives the optical signal to/from fiber optic lines. A transponder may be characterized by its data rate and the maximum distance the signal can travel. First generation optical and electrical transponders using PAM-4 line side modulation commonly have a non-return-to-zero (NRZ) encoded host interface and a PAM-4 line interface. NRZ is commonly used in serial communications. NRZ tracks the values being sent; therefore, an idle state, where all the bits are the same value, leaves the signal at the same level during the idle time. The transponder may be connected between an electrical telecommunication system and an optical telecommunications system. The NRZ encoded host interface of the transponder may be connected to the electrical telecommunication system, and the PAM-4 line interface of the transponder may be connected to the fiber optic line to transmit or receive signals on the fiber optic line.
These type of optical and electrical transponders include multiplexing/de-multiplexing and PAM-4 modulation/demodulation functionality in order to convert from lower speed NRZ host interface lanes to higher speed PAM-4 encoded line interface lanes. Typically, the number of line interface lanes is half the number of host interface lanes. Normally, the host interface is electrical. The line interface can be electrical or optical (multi-fiber or multi-wavelength). An example is a 400 Gbit/s transponder with a host interface consisting of sixteen electrical 25 Gbit/s lanes and a line interface consisting of eight 50 Gbit/s PAM-4 encoded wavelength lanes.
Defined PAM-4 encoded line interface test patterns are used to test these types of transponders. Examples of the defined PAM-4 test patterns are PRBS13Q, PRBS31Q and linearity test patterns. PRBS stands for pseudorandom binary sequence. Q stands for quatranary. One or more of these defined test patterns may become standardized, such as proposed by IEEE 802.3bs, and are used for key measurements to measure performance parameters, such as bit error rate, optical performance parameters, receiver sensitivity and jitter. In addition to these key measurements performance analysis and troubleshooting can be greatly improved by being able to stimulate, detect and analyze signals at the PAM-4 symbol level, i.e. by being able to perform PAM-4 symbol stimulus/analysis rather than NRZ bit level stimulus/analysis.
The difficulty in testing the transponders is that the test instrument does not have direct access to the PAM-4 encoded line interface of the transponder. The test instrument is connected to the electrical NRZ encoded host interface of the transponder, so the test instrument cannot provide PAM-4 encoded test patterns directly to the PAM-4 encoded line interface of the transponder. Furthermore, the transponder internally converts between NRZ and PAM-4 encoding. Such conversion may encompass NRZ lane skew compensation, NRZ bit level muxing/demuxing, Gray encoding/decoding and PAM-4 modulation/demodulation. These conversion processes are complex and can be different from one transponder implementation to the next. In addition, the conversion processes may be dependent on certain start conditions like relative bit multiplexer phase and relative NRZ lane skew. As a result of the internal conversion processes, it is difficult to control and analyze the transponder's line interface PAM-4 signals via its NRZ host interface.